In recent years, there is an increasing demand for high scratch and impact resistant components for mobile electronic devices typified by smartphones. Particularly, exterior components of mobile electronic devices have a thin plate shape with a thickness of 2 mm or less and must withstand impact such as dropping impact. Therefore, a material having particularly high impact resistance is required for these exterior components.
Materials used for the exterior components may include metals, resins, glasses, etc., and glass materials having scratch resistance and high designability are widely used. The glass materials currently used are tempered glass strengthened by ion exchange. In such tempered glass, a strengthened layer of about several tens of micrometers is formed on the surface of the glass by ion exchange to generate compressive stress on the surface, and propagation of cracks is thereby prevented. However, since the mechanism of strengthening of the tempered glass is due to the strengthened layer, the tempered glass has the following problems, and further improvement is required.
(1) If a crack propagates across the strengthened layer, the tempered glass may break at once.
(2) The Vickers hardness of the glass itself is about 600. Therefore, the glass is easily scratched by contact with metal, concrete, etc., and scratches formed during use may cause a significant reduction in strength.
(3) The tempered glass cannot be machined after the strengthening treatment.
(4) Even when the tempered glass is a chemically strengthened glass, the presence of a machining scratch on an end face may cause a reduction in end face strength.
(5) When the tempered glass breaks, fine sharp broken pieces are formed.
Ceramics are excellent in heat resistance, wear resistance, and corrosion resistance and are therefore widely used for industrial component applications. Particularly, zirconia ceramics have high strength, high toughness, high hardness, and scratch resistance, and their design can be easily improved by coloring. Therefore, the zirconia ceramics are being increasingly used for watch components etc.
The use of ceramics for exterior components of mobile electronic devices etc. is also contemplated. Particularly, when an exterior component of a mobile electronic device is increased in thickness in order to improve impact resistance, the weight of the component becomes large, which is not practically advantageous. When the component is reduced in thickness to reduce the weight, the resistance of the component to impact of falling, collision, etc. becomes insufficient, and the component easily cracks and becomes unusable.
To improve the impact resistance of a ceramic component, methods similar to so-called laminated glass has been proposed (see, for example, Patent Literatures 1 and 2). Specifically, a ceramic plate is joined to a fiber-reinforced plastic to prevent a missile such as a shell or a bullet from piercing the ceramic component. Patent Literature 1 reports a component in which alumina with a thickness of 8 mm and silicon carbide are joined to a fiber-reinforced plastic.
An exterior component of a mobile electronic devices etc. must have resistance to cracking caused by a free falling impact object with a weight close to the own weight of the device (about 100 g). In the conventional method, a thick ceramic component must be used, and this results in an increase in the weight of the component, so that the product cannot be used for a mobile exterior component. To reduce the weight, improvement of cracking resistance must be achieved using a thin plate. However, there has been no impact resistant component that is used for a zirconia plate having a thickness of 2 mm or less and can improve resistance to cracking caused by the impact of falling, collision, etc., and there has been no method for producing such an impact resistant component.
Patent Literature 3 describes a watch cover glass produced by joining sapphire to an inorganic glass. The purpose of this cover glass is, by disposing the high-hardness sapphire on the surface of the watch cover glass, to improve its scratch resistance. However, high impact resistance required for the mobile electronic device applications cannot be achieved by using this method.
Therefore, there has been no impact resistant component that is used for a zirconia sintered body plate having a thickness of several millimeters and has a sufficient white color tone and improved resistance to cracking caused by the impact of falling, collision, etc., and there has been no method for producing such an impact resistant component.